Follow through, overlapping action and drag
follow through give the impression that characters follow the laws of physics. it means that separate parts of a body will continue moving after the character has stopped. “Overlapping action” is the tendency for parts of the body to move at different rates (an arm will move on different timing of the head and so on). A third related technique is “drag”, where a character starts to move and parts of him take a few frames to catch up.
Slow in and slow out
The movement of the human body, and most other objects, needs time to accelerate and slow down. For this reason, animation looks more realistic if it has more drawings near the beginning and end of an action, This principle goes for characters moving between two extreme poses, such as sitting down and standing up, but also for inanimate, moving objects, like the bouncing ball in the above illustration.
Most natural action tends to follow an arched trajectory, As an object’s speed or momentum increases, arcs tend to flatten out in moving ahead and broaden in turns. In baseball, a fastball would tend to move in a straighter line than other pitches; while a figure skater moving at top speed would be unable to turn as sharply as a slower skater.
Adding secondary actions to the main action gives a scene more life, and can help to support the main action. A person walking can simultaneously swing his arms or keep them in his pockets, he can speak or whistle, or he can express emotions through facial expressions. The important thing about secondary actions is that they emphasize, rather than take attention away from, the main action.